Self-heating degradation of n-type metal-induced laterally crystallized polycrystalline silicon thin-film transistors is systematically investigated under various stress powers. A two-stage degradation behavior with turnaround effect at the initial stage is characterized. The initial degradation stage is related to breaking of weak Si-H bonds. The floating-body effect by released hydrogen ions is responsible for the observed back-shift of the transfer curve during the initial stress. On the other hand, the normal degradation stage occurs by breaking of strong Si-Si bonds and trap generation at grain boundaries (GBs) and the gate oxide/channel interface. Our model is supported by observed different activation energies related to two degradation stages and a direct observation of the continuous increase in GB trap density during the normal degradation. Furthermore, during the normal degradation stage, an anomalous continuous field-effect mobility increase along with its V, dependence shift is first observed. It is clarified that this behavior is not a true channel mobility increase, but a consequence of stress-related trap generation.